You can write up to 512 bytes at a time with the Write command. If num is omitted, then 512 bytes will be expected.

After the command is received, num (or 512) bytes are expected to be received. Data is received raw (i.e. no escape sequences are required).

By default, there is no time-out for how long it takes to send the data. This means that the uMMC will wait indefinitely for all the data to be sent.

If the value of the Write Time-outsetting is greater than 0 (zero), then if the time taken between sending bytes exceeds that value (in tens of milliseconds), the Write command will terminate, write the accepted bytes to the file, and return to the command prompt. No error will be returned.